It is standard practice in most iron foundries to perform chill tests On heats as a running documentary of the structural quality of gray and ductile irons prior to pouring.

The test is a readily available aid to determine the need for and the amount of inoculation as the melt is prepared for tapping. To the practiced eye, the conditions revealed in a chill test give a graphic readout of the mews conditions.

Chill testing is a procedure for approximating the graphitizing tendency in iron. A test sample of the iron is taken and poured into a core sand mold or test block. The sample is removed from the mold solidification and is broken, allowing the inspector to view the fracture and measure the chill structure. The depth of the chill is measured in increments of 1/32 of an inch.

The wedge test uses a specifically dimensioned wedge that is cast in a core mold, cooled and then broken. The test indicates where the white iron and gray iron meet. Results are read according to a standardized interpretation of the size of and location of that junction.

The chill plate test is better adapted to irons with silicon contents exceeding 2.5% and carbon at or above 3.5%. As its name implies, the chill plate test specimen is poured against a chilled plate that is usually made of cast iron, graphite, or, alternately, water-cooled steel or copper plates. The molded chill contains a notch to facilitate breaking of the casting. Chill Test Factors

Several factors need to be considered when standardizing gray iron chill test results. Two key ones are pouring practices and test specimen cooling.

Pouring Practices-The pouring temperature of metal must be controlled within fairly close limits because variations tend to skew quality control results. With the same base iron and a given chill test specimen, for instance, a low pouring temperature will result in a greater chill value than will result from a high pouring temperature.

The molten metal sample should be representative of the iron being tested. The iron should be mixed before sampling. In the case of inoculated iron, proper mixing takes on added importance to ensure uniform dispersion of the inoculant before testing.t

The sampling ladle should be refractory lined and should hold at least 5 lb of metal. This is important in order to prevent excessive cooling before the test specimen is poured. The sampling ladle should be hot and the well-stirred test specimen poured as quickly as possible. This assures that the chill sample closely represents the molten metal in the furnace.

In testing cupola iron, it is not considered accurate practice to take chill test samples from the cupola spout because cupolas do not always melt uniformly. The molten metal can vary in composition from charge to charge. It is better practice to collect and mix the metal from several charges before taking chill test specimens.

Test Specimen Cooling-It is well known that graphite precipitation and primary carbide formation in gray cast iron occur during the solidifiction stage. The rate at which solidification occurs determines whether the carbon will be present as graphite flakes or in the combined form known as cementite.

The maximum chill value of the iron is attained when solidifiction is completed and no more primary carbide or cementite is formed. Thus, a chill test specimen can be quenched as soon as it is completely solid. However, the heavy end of e sample must be immersed in the quench bath first to prevent cracking of the specimen's apex.

If time is available, chill test specimens can be air-cooled until they are a dull red or to a colder stage before quenching. This will yield a more pronounced demarcation between the chilled and gray portions of the chill specimens because the iron Matrix difference and values will be easier to read.

Cooling in air after solidification favors the formation of pearlite or ferrite in a cast iron matrix. A fast cooling rate forms a martesitic or very hard matrix structure. The fracture of a wedge specimen with a martensitic matrix structure will have a somewhat lighter gray color and sometimes will be more shiny than a chill specimen of the same iron with a pearlitic or ferritic matrix structure. Cupola Handbook, 5th Edison, American Foundrymen's Society, Inc. (1984).

COPYRIGHT 1991 American Foundry Society, Inc.
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